The present invention relates to improvements in closure systems for open ended osteosynthesis apparatus used to correct orthopedic deformities and, in particular, for use in spinal osteosynthesis.
Surgically implanted spinal osteosynthesis apparatus often include rods which are secured along at least a portion of the spine by a system of hooks, bone screws, including sacral screws and pedicle screws and transverse connectors for the purpose of stabilizing and adjusting spinal alignment.
The hooks, bones screws and transverse connectors or other spinal surgical implants may be of a closed end type or open end type. In a closed end type a bore, sized to receive a spinal rod, extends through the head of the implant. In an open end type the head of the hook, screw or connector generally incorporates a U-shaped groove sized to receive a spinal rod. A spinal rod is positioned in the U-shaped groove and a cap or other closure system is then secured to the head of the implant to enclose the spinal rod in the U-shaped groove.
In a relatively simple spinal osteosynthesis apparatus, open ended screws are screwed into the pedicle portion of the vertebra at desired locations and a spinal rod is then positioned in the U-shaped channel in each bone screw. A cap is then secured to the head of the screw to close off the U-shaped groove and to secure the spinal rod therein and a set screw extending through the cap is tightened against the rod to fix the translational and rotational relationship of the rod within the closed off U-shaped channel. The rods are bent or shaped, before or after insertion into the U-shaped channel, to maintain the spine in a desired configuration, to provide support to the spine and to exert desired corrective or stabilizing forces on the spine.
A slightly more complicated system uses transverse connectors in association with the bone screws to secure the spinal rods. The transverse connectors include an arm and a head. The head has a U-shaped channel formed therein. The arm of the connector is secured to the pedicle screw and the spinal rod is then inserted into the U-shaped channel of the transverse connector which is then capped off. A set screw secured within a threaded bore in the cap is then tightened to fix the relative position of the rod within the spinal rod bore.
One type of open end bone screw is shown in the Cotrel U.S. Pat. No. 5,005,562. The device in the Cotrel patent has threaded interior surfaces on the two upright branches that form the rod receiving channel therebetween and which receive a threaded set screw having a rod engaging point and outer ring. The set screw in Cotrel is tightened against the rod by advancing the set screw along the threads. However, this system has limitations. In particular, the ability of the set screw of Cotrel to grip and hold the rod is heavily dependent on the torque applied to the set screw during installation. Unfortunately, the torque is limited because too much torque will cause the branches to spread, thereby allowing the set screw to loosen and the implant to fail. Such failure can also occur when forces are applied to the implant during use, such as at time of muscular stress or during accidents when the back is jolted. To try to overcome this problem associated with the Cotrel device, the implant branches and set screw are increased in size to add strength and/or a retention ring is placed around the outside of the branches to reduce the likelihood of expansion. However, the strengthening adds substantial bulk to an implant and a ring adds bulk and complexity to the implant. In implants, it is important to try to reduce bulk rather than add to it, as it is desirable for the implants to be as low profile as possible.
In U.S. Pat. No. 5,562,663, I disclosed a cap for closing off a U-shaped channel of a bone screw with a rod inserted therein wherein the cap mates with the branches on opposite sides of the screw to prevent the branches from expanding radially outward. The cap incorporates a pair of curved tongues extending from the sides of a central portion which are adapted to be received in curved channels in opposed branches of the screw. The cap is rotated through an arcuate motion for insertion between the branches of the bone screw. Some practice is necessary for surgeons to familiarize themselves with the appropriate angel required to initiate insertion of the cap. Although use of the curved tongue and channel configuration can be advantageous in that the curved entry reduces the amount of open area behind the screw to permit insertion, the curved channels are relatively expensive to produce.
There remains a need for an improved closure system for an open ended spinal surgery implant which is relatively small and easy to install, which can be used to reliably fix the position of a spinal rod within the U-shaped channel of the implant and which is relatively inexpensive to manufacture.